2. Electrical Method
Electrical resistivity method is based on the difference in the electrical
conductivity or the electrical resistivity of different soils. Resistivity is
defined as resistance in ohms between the opposite phases of a unit cube of a
material.
ρ=(
𝑅𝐴
𝑙
)
ρ is resistivity in ohm-cm,
R is resistance in ohms,
A is the cross sectional area (cm 2),
L is length of the conductor (cm).
3. The resistivity values of the different soils are listed in table 1.4
Material Resistivity ( Ω-cm)
Massive rock > 400
Shale and clay 1.0
Seawater 0.3
Wet to moist clayey soils 1.5 - 3.0
Table 1.4 : Resistivity of different materials
4. Principle
All electrical methods are based on the fundamental fact that
different materials of earth’s crust methods posses widely different
electrical properties. Resistivity, Electrochemical activity and
dielectrical constant are some of these properties that are generally
studied through this methods. Result obtained from such studies when
interpreted properly give sufficiently useful clues regarding the nature
and make up of the subsurface materials.
5. Two common methods
1.Equipotential Methods:
In this method 2 primary electrodes are inserted into the ground,6-7
metres apart from each other, across which current is introduced. The
position of these primary electrodes remains fixed in the subsequent
investigations.
Potential between these primary electrodes is determined with the
help of two search electrodes and points of equal potential found out
along the entire region under investigation, which are joined to get
equipotential lines.
6. Under normal conditions, i.e when the material below is of uniform
nature, the equipotential lines would be regular in character. But in
cases when the material below is not of uniform character i.e ,it
contains patches of high or low conductivity, the equipotential lines
would show clear distortions or irregularities which would include the
probable locations of rock masses of different characteristics.
7. 2.Resistivity Method:
It is similar to equipotential method but in this case it is the
resistivity of the material of the subsurface which is determined from
which important interpretations are made. Here also, a known current
is introduced through two electrodes- the current electrodes, which
are inserted at some distances apart from each other. Potential
gradient is then measured directly with the help of two or more
potential electrodes placed at proper distances within the two outer
current electrodes.
8. A typical resistivity meter
A resistivity meter consist of both a voltmeter an a currentmeter (ammeter).
Most system report the ratio V/I instead of each one seperatly.
The resistance can then be converted into resistivity using geometrical
parameters based on the type of array.
Most modern resistivity system typically utilize atleast 4 electrodes.
About 70% of the current applied by two electrodes at the surface stays
within a depth equal to the seperation of the electrodes.
In this method, the electrodes are driven approximately 20cms in to the
ground and a dc or a very low frequency ac current of known magnitude is
passed between the outer (current) electrodes, thereby producing within the
soil an electrical field and the boundary conditions.
9.
10. RESISTIVITY-HOW DO WE MEASURE IT???
Apply a non potential difference(measured with voltmeter) to a circuit with a
resistive material of known length and cross sectional area.
Then measure the current with ammeter.
This gives the resistance R.
11. The spacing of this current and potential electrodes is of vital
importance in this method. A number of arrangements have been
suggested of which WENNER’S arrangement is followed quite commonly.
In this arrangement , the potential electrode are placed at a distance
1/3 ‘a’, where ‘a’ is the total distance between the current electrodes.
Resistivity is then calculated by the formula:
ρ=2𝜋
𝑑𝑣
𝐼
Where ρ=resistivity in ohm-m, v=Potential difference, I=current in
ampere, d=
1
3
‘a’, where ‘a’ is total distance between outer electrodes.
12. For simple sounding, a Wenner array is used as shown in fig. 1.16. Then, the
resistivity is given as,
ρ=(
2π𝑅𝑎
𝑙
)
a is the spacing between the electrodes.
Fig. 1.16 Wenner arrangement
13. Advantages of Electrical Method
It is a very rapid and economical method.
It is good up to 30m depth.
The instrumentation of this method is very simple.
It is a non-destructive method.
14. Disadvantages of Electrical Method
It can only detect absolutely different strata like rock and water.
It provides no information about the sample.
Cultural problems cause interference, e.g., power lines, pipelines, buried
casings, fences.
Data acquisition can be slow compared to other geophysical methods,
although that difference is disappearing with the very latest techniques.
15. Resistivity used in:
Map faults
Map lateral extent of conductive contaminant plumes
Locate voids
Map heavy metals soil contamination
Delineate disposal areas
Map paleochannels
Explore for sand and gravel
Map archaeological sites
16. Methods
1.Self Potential methods:
All those electrical methods that involve measurement of natural
electrical potential of the subsurface rocks are included under this
heading. The natural potential may be due to electrochemical
reactions between the solutions and the surrounding subsurface rocks.
These reactions are not always of the same order throughout the
dimensions of the rock masses thereby creating a potential difference
and conditions for flow of current from one end to the other end.
Elongated ore bodies of magnetite and pyrite etc are easily delineated
by this method.
17. 2.Potential Drop methods:
This include a variety of methods in which electrical current is
artificially introduced from an external source at certain points and
then its flow through subsurface materials recorded at different
distances. The behaviour of the flow lines of the electrical current is
directly related to the nature of the subsurface materials.
18. The depth of penetration of electrical current in these investigations is broadly equal
to d although there are many attached to this generalization.
The resistivity method envisages interpretation of the qualitative as well as
quantitative characters of the sub surface materials which are governed by two basic
principle:
(i) If material below is of uniform nature , the resistivity values would be of
regular character.
(ii) If the material is non-uniform, that is ,it consist of layers or masses of
different character ,then these would be indicated irregularities or anomalies in the
resistivity values. The depths of at which these anomalies occur can be calculated
and also the nature of the sub surface material broadly understood.
19. In common practice, the resistivity of a given area is measured by
gradually increasing the distance between the electrodes and by
changing the directions of the profiles . Results are plotted on a graph
with resistivity and depth as main factors. An abrupt change in the
resistivity curve(rise or fall) would often suggest a change in the
nature of the material corresponding to that depth. The existence of
specially low zones such as aquifers, fractured zones, buried valley
etc. may be indicated even in linear traverse keeping the spacing
between the electrodes constant along the entire traverse.
20. Applications
(a)In Prospecting: The electrical methods have been successfully
employed in delineation of ore bodies occurring at shallower depths.
For such surveys at great depths, these are not of much help.
In table 1 some typical value-ranges of resistivity are given. As may be
seen, rocks exhibit a great variation ranging from as high resistivity as
> 10^5 ohms-meters in igneous rocks to as low as less than 1 ohm-m
for clayey marls.
(b)In Civil Engineering: Resistivity methods have been widely used in
engineering investigation for determination of
(i)Depth to the bed rock: as for instance, in important projects like
dams, buildings and bridge foundations, where it would be desirable
that the structure should rest on sound hard rocks rather than on
overburden or soil.
21. (ii) Location of geological structures: like folds; buried valleys,
crushed and fractured zones due to shearing and faulting (Fig 22.3)
(iii) Location of Aquifers: and other water bearing zones which could
be easily interpreted on the basis of known resistivity values of
moistures rich rocks and dry rocks.
22. Some more Applications
Characterize subsurface hydrogeology
Determine depth to bedrock/overburden thickness
Determine depth to groundwater
Map stratigraphy
Map clay aquitards
Map salt-water intrusion
Map vertical extent of certain types of soil and
groundwater contamination
Estimate landfill thickness
23. ELECTRICAL RESISTIVITY METHOD FOR
GROUNDWATER INVESTIGATION
Electrical resistivity methods of geophysical prospecting are well established
and the most important method for groundwater investigations. Groundwater,
through the various dissolved salts it contains, is ionically conductive and
enables electric currents to flow into the ground. Consequently, measuring
the ground resistivity gives the possibility to identify the presence of water,
taking in consideration the following properties:
• a hard rock without pores or fracture and a dry sand without water or clay
are very resistive: several tens thousands ohm.m
• a porous or fractured rock bearing free water has a resistivity which
depends on the resistivity of the water and on the porosity of the rock (see
below): several tens to several thousands ohm.m
• an impermeable clay layer, which has bound water, has a low resistivity:
several units to several tens ohm.m
• mineral orebodies (iron, sulphides, …) have very low resistivities due to
their electronic conduction: usually lower or much lower than 1 ohm.m
24. GROUNDWATER DETECTION
To identify the presence of groundwater from resistivity measurements, one
can look to the absolute value of the ground resistivity, through the Archie
law: for a practical range of fresh water resistivity of 10 to 100 ohm.m, a
usual target for aquifer resistivity can be between 50 and 2000 ohm.m. Most
of the time it is the relative value of the ground resistivity which is
considered for detecting groundwater: in a hard rock (resistant) environment,
a low resistivity anomaly will be the target, while in a clayey or salty
(conductive) environment, it is a high resistivity anomaly which will most
probably correspond to (fresh) water. In sedimentary layers, the product of
the aquifer resistivity by its thickness can be considered as representative of
the interest of the aquifer. However, electrical methods cannot give an
estimation of the permeability but only of the porosity. The contrast of
resistivity between a fresh water and a salted water (coming from a sea
intrusion for instance) is high and the depth of the water wedge is usually
well determined with electrical methods.
